1. Field of the Invention
The invention relates to a rotor with folding blades for the rotary wings of an aircraft, and in particular for a helicopter main rotor.
The rotor with folding blades according to the invention is of the type comprising a hub, secured to a rotor mast, generally at one end of the mast, intended to be rotated about an axis of the rotor, and to which each blade of the rotor is connected by a connecting member which is substantially radial with respect to the axis of the rotor, the connecting member being itself connected to the hub by retaining and articulating means allowing angular excursions of the corresponding blade with respect to the hub at least in terms of pitch about a longitudinal pitch-change axis of the blade, each blade being mounted so that it can pivot with respect to the corresponding connecting member about an axis of folding, so that it can be moved, when the rotor is stopped, between two positions, one of which is a flight position, in which the blade is substantially radially in line with the connecting member, and the other of which is a folded position, in which the blade is pivoted about the axis of folding to one side of the connecting member towards the rear of the aircraft.
When the latter is a helicopter, the blades of its main motor may thus be folded backwards, on each side of the rear part of the fuselage and of the tail boom of the helicopter, with the possible exception of one of the blades of the rotor which, when the rotor has an odd number of blades, can be aligned on its pitch axis with the longitudinal axis of the helicopter and directed towards the rear of the latter without needing to be folded.
2. Description of the Prior Art
In general, the blades of a helicopter main rotor are folded in order to reduce the overall size of the helicopter, in typical usage scenarios, especially in use on land to facilitate and optimize the storage of the helicopter in a hangar, with a particular importance in the case of small hangars in cold countries and, in military applications, to make it easier to hide the helicopter under camouflage or under the cover of trees, and to make it easier to load the helicopter into the hold of an aeroplane or, when used on board a ship, to fit inside the lifts of aircraft carriers and/or to be compatible with a hangar adjacent to the helicopter deck on the ship.
Folding can be done manually, by operators using implements for moving and positioning or lashing down the blades, such as poles fitted with grippers for lifting the blades, pivoting them and lashing them down to the tail of the helicopter, or automatically, without the assistance of people but by operating maneouvering actuators, for example electro-hydraulic or electromechanical actuators mounted on the rotor, and folding and deployment will be either manual or automatic depending, especially, on the mass of the blades and on their height relative to the ground (which determines the size and weight of the poles and the effort the operators will have to exert), the external conditions, such as wind speed and/or movements of the deck of the ship, or alternatively the time needed to clear a landing or deck-landing area of a first helicopter in order to receive a second one.
The criteria governing a folding-blade rotor are essentially the width of the rotor with the blades folded, the possibilities for lashing the folded blades to the back of the fuselage and/or to the tail boom of the helicopter, and the fitting, during folding, of the forward-pointing blades of the stationary rotor under the non-folding members of the backward-pointing blades, and in particular their members for connecting them to the hub, when the rotor has more than three blades.
Indeed, folding the blades of a three-blade rotor does not pose any particular problems, as one blade is directed backwards, over the tail boom, along the longitudinal axis of the helicopter, and each of the other two blades is folded backwards on each of the two sides of the tail boom, encountering no obstacles in their path.
By contrast, difficulties mount when the number of blades increases beyond four blades, both as regards the width of the rotor with the blades folded and as regards lashing all the blades down to the tail boom, it being possible for additional constraints to arise out of the proximity of upper covers, especially of the turbo engine unit or of jet dilution/deflection devices, or alternatively of folding the rear part of the tail boom with the counter-torque rotor, for helicopters on board ships.
A folding-blade rotor of the type introduced hereinabove, and which is folded automatically, is known from EP-A-0,057,053. In this document, the axis of folding of each blade is offset laterally with respect to the pitch axis of the blade and perpendicular to this pitch axis and vertical, like the axis of the rotor. The blades thus pivot about axes which are parallel to the axis of the rotor and this, because of the flexural deformation of the blades under the effect of their own weight and also, sometimes, of the wind and/or movements of the ship, has the drawback that it leads to interference, during folding, between the front blades and the rear blades or other members of the rotor such as the pitch rods connecting the swashplates to the blades.
In an attempt to overcome these drawbacks, it has already been proposed that the path and final position of each blade, while it is being folded, be defined by choosing an appropriate inclination for the axis of folding in a plane perpendicular to the pitch axis of the blade so as to allow blades coming from the front to pass underneath the rear blades, which should be higher up, because the rotor mast is generally inclined towards the front of the helicopter.
In a rotor with manual folding of the blades, each blade is generally held on its connecting member by means of two pins which are symmetric with respect to the pitch axis and, depending on whether the blade extends to the right or to the left, one of these two pins, which is removable, is removed so that the blade can be pivoted about the other pin which is on the side towards which the folding movement occurs. In order to obtain the desired path for each blade, one solution consists in inclining the pivot pin in a plane perpendicular to the pitch axis of the blade by rotating its member for connection to the hub about its longitudinal pitch axis, but this results in natural conflict between the front-left and front-right blades, the connecting members of which have to be rotated in opposite directions.
Other drawbacks of this known solution are that it is not clear how to find a position for the swashplates which is satisfactory for all of the blades while at the same time avoiding, in particular, having to disconnect one or more pitch rods, this being to avoid wasting time and the risk of incorrect reconnection for the flight configuration; furthermore when the means of holding and articulating to the hub provide a torsionally elastic pitch articulation, for example when these means comprise an elastomeric laminated spherical stop, or a bundle or a torsion blade, these articulation means are stressed for long periods of time by permanent twists which differ from blade to blade.
In the case of rotors in which the blades are folded automatically, it is not permitted to have to disconnect the pitch rods, and the bulk of the blade-folding mechanism and possibly of the associated mechanisms for locking the blade in terms of pitch, or even in terms of flap and/or drag for folding, near to the axis of folding of each rear blade, leads to a search for even lower positions for the front blades than are found in the case of a rotor which is folded manually. Thus, for the same initial pitch of all the blades, each member for connecting a blade to the hub needs to be designed with the desired inclination of the corresponding axis of folding in a plane perpendicular to the pitch axis.
A drawback of a solution of this kind is that the greater inclination for the front blades than for the rear blades, and the symmetry between the blades on the right and the blades on the left unavoidably means that it is necessary to have as many different connecting members as there are blades on the rotor.
Furthermore, whether the rotor blades are folded manually or automatically, the choice of appropriate angles of inclination for the axes of folding in planes perpendicular to the pitch axes of the blades leads to blade paths with steep slopes and to the folded blades presenting a large area exposed to the wind, and this limits the foldability to low wind speeds, and a complicated procedure for manual or automatic folding requiring the use of implements for manoeuvring and/or positioning the blades, such as the aforementioned poles, or a need to operate the hydraulic blade-folding actuators at high pressure.
The problem underlying the invention is that of overcoming at least one of the aforementioned drawbacks, and preferably a number of them simultaneously, in the case of manual or automatic folding, and to provide a rotor with folding blades which is better suited to the various practical requirements than those known at present.